Finely grate the zest of 2 lemons. Squeeze enough juice to make ½ cup.

In a mixer, preferably fitted with a paddle attachment, cream the butter and Swerve together. Beat in the lemon zest and juice and the eggs, one at a time. Beat until well combined.

Pour the mixture into a medium saucepan and cook over low heat, stirring constantly, until thickened, about 10 minutes. Remove from heat and let cool. Can be stored in the refrigerator, covered, for a week.